1. Field of the Invention
The present invention relates to glasses colored in yellow, and more particularly to colored glasses in which a yellow color of high excitation purity has been developed, and which have an extremely low transmittance in the vicinity of 500 nm or shorter. Because of the above-described properties, the glasses can be utilized for various types of art glass and decoration glass, optical filters and the like.
2. Related Art
In order to color glass in yellow, a colored ion such as Cr.sup.6+, V.sup.5+, Ni.sup.2+, Ti.sup.2+ or Ce.sup.4+ has been conventionally used. The coloring of glass with such a colored ion is greatly affected by the ion concentration, the atmosphere under which the glass is melted, and the glass-melting temperature. In particular, the valence of the ion varies with the glass-melting temperature, and, as a result, the absorption wavelength of the glass tends to change. It is thus technically difficult to develop a stable yellow color in glass by using a colored ion. Moreover, it is considerably difficult to develop a pure yellow color of high excitation purity in glass by color development of a colored ion.
On the other hand, there is a method in which a colloid is dispersed in glass to give color thereto. Ag colloid and CdS colloid have been known as colloids which can color glass yellow. In the case where Ag colloid is used for the coloring of glass, a stable color cannot be given to the glass unless the optimum temperature for the formation and growth of metallic crystal nuclei and the thermal reduction reaction are strictly controlled. In the case where CdS colloid is used for the coloring of glass, a clear yellow color can be developed in the glass. However, the CdS colloid, a coloring material, may be volatile at a high temperature at which the glass is melted. In addition, it is difficult to adjust the colloid particle diameter of the CdS colloid by controlling the conditions under which the glass is cooled or heat-treated, so that the spectral properties of the colored glass obtained are readily changeable. Moreover, during the step of melting of the glass, the CdS colloid vaporizes cadmium, which is noxious, so that a cadmium-removing apparatus is needed. It is thus difficult to obtain glasses colored in yellow by the method using CdS colloid on a mass-production scale.
Some cases wherein iodine or an iodine compound is added to glass have been known. For instance, Japanese Laid-Open Patent Publication No. 11248/1985 describes a technique in which an iodine compound is used as a constituent of chalcogenide glass. However, although the chalcogenide glass has transmission in the infrared and RF ranges of the spectrum, it does not have transmission in the visible light range of the spectrum. It is therefore understood that this technique is not for obtaining glasses colored in yellow.
Furthermore, Japanese Laid-Open Patent Publications Nos. 109242/1980 and 215524/1988 disclose a method in which glass is prevented, by the addition of iodine, from foaming during the steps of melting and forming. This method is to impart enhanced transmission in the short wavelength range (in the vicinity of 200-300 nm) of the spectrum to the glass, and it is not a technique for imparting a yellow color to glass.